G’day OGs
I have this code as follows:

def create(conn, %{"a" => a_params})  do
    with true <- someClause(),
         a_params <- someClause(a_params),
         :ok <- Enum.reduce_while(get_in(a_params, ["b"]), %{}, fn {_key, %{"c" => c , "d" => d}}, _iteration ->
          case Module.check(c, d) do
            :ok -> {:cont, :ok}
            {:error, "..."} ->
                {:halt, {:error, "..."}}
          end
         end), do
          #do things
    else
        {:error, "..."} -> throw errors
    end
end

In some tests, %{b} in a_params would be nil
So reduce_while/3 would fail
How can I prevent reduce_while/3 from executing if my map is nil along the above condition ?.
Thank you.